It is known to insert a tubular synthetic resin piece having a heat resistant resin layer into a mouth of a container principally formed of polyethylene terephthalate, so that the mouth of the container will have two or three layers to improve the heat resistance of the mouth of the container.
Most of the heat resistant layer of the mouth formed by the piece is formed on the surface of the mouth since the piece is inserted into a cavity which molds the mouth, after the resin which forms the container body is poured.
When forming a multilayer mouth of a container using the insert as described above, the piece to be inserted has already been solidified, and therefore the in adhesiveness between the piece and the poured molten resin may be poor. In addition, since the properties of the piece and the molten resin are different from each other, they tend to peel off when an external force is applied even if they are integral when molded. As the case may be, they cannot withstand the external force due to the peeling, and as a result, the piece becomes cracked, failing to achieve the object as a container.
In view of the foregoing, it has been proposed that the piece be inserted into the mouth of the container to be embedded therein as described in Japanese Patent Application Laid-Open No. 61-259946. Since the entire insert piece is covered by the container forming resin, a high load such as torque acting on the piece from the exterior is less than that when the piece is present on the surface. Even if peeling occurs, the contour of the mouth of the container will be maintained, with the result that there is no obstacle to the specification of the mouth.
However, it is very difficult to insert the piece into the middle of the mouth of the container. There is a special problem in that the position of the piece must be mechanically supported within the mold against the pouring pressure of the molten resin. Where the side of the insert piece is supported by means of a pin or the like to stand away from the mold surface, if there are insufficient supporting points, the location of the piece will deviate. Furthermore, a pin trace remains in the molded mouth, and therefore the resultant article is not desirable. Additionally, the construction of the mold is complicated as compared with a conventional mold. This results with loss of many molds and increased costs. It is therefore necessary to further improve the heat resistance of the mouth of the container with a piece used as an intermediate layer.